The short answer
Generally, no. Finite (160 mm) objectives are designed to form an image a set distance up the microscope tube on their own. Infinity-corrected objectives send out parallel light and rely on a separate tube lens inside the microscope to form the image. Put one type on a microscope built for the other, and you’ll usually get one or more of these:
- trouble reaching focus;
- objectives that are no longer parfocal;
- the wrong magnification;
- a softer image, especially at high power.
A low-power objective used the “wrong” way can give a viewable image. That’s fine for a curious look, but not something to rely on. For good results, keep to objectives designed for your microscope’s optical system.
How a 160 mm finite system works
In a finite microscope, the objective takes light from the specimen and focuses it directly into a real, magnified image inside the tube. This is the intermediate image, which the eyepiece then magnifies. The objective is designed for one specific distance to that image, set by the mechanical tube length: the distance from the objective’s mounting shoulder to the top of the eyepiece tube.
A 160 mm mechanical tube length was standardized by the Royal Microscopical Society and was used across much of the industry for roughly a century. It’s still common on student, amateur and many older laboratory microscopes. Objectives designed for it are usually engraved 160.
The weakness of the design is that anything added to the light path between objective and eyepiece changes the effective tube length. Examples include a reflected-light illuminator, a polarizing intermediate or a camera port. Manufacturers had to add correcting optics to such accessories to compensate.
How an infinity-corrected system works
In an infinity-corrected microscope, a specimen at the objective’s focal point produces a beam of parallel light leaving the objective. No image forms until that light reaches the tube lens further up the body, which focuses it into the intermediate image for the eyepieces.
The space between objective and tube lens is often called “infinity space”. Filters, polarizers, DIC prisms and fluorescence illuminators can go there with little effect on focus or image quality. That flexibility is the main reason research microscopes moved to infinity optics.
In an infinity system, the objective’s magnification depends on the tube lens:
Magnification = tube lens focal length ÷ objective focal length
Tube lens focal lengths aren’t standardized between makers. Commonly cited values are about 200 mm for Nikon and Leica, 180 mm for Olympus/Evident and 165 mm for Zeiss. So an infinity objective marked 40x gives 40x only with its intended tube lens. It may give a different magnification on another brand’s body.
How to tell which type you have
Look at the barrel. A typical marking reads something like 40x / 0.65 160 / 0.17 or Plan 40x / 0.65 ∞ / 0.17.
- 160 (or occasionally another number such as 170) after the slash means a finite objective for that tube length.
- ∞ means infinity-corrected.
- The number after it, usually 0.17, is the coverslip thickness the objective is corrected for. It has nothing to do with the finite or infinity question.
Very cheap objectives sometimes have no system marking. In that case, check the microscope’s documentation or ask the seller. If the microscope itself is old or basic, finite is the safer assumption. For a full guide to barrel markings, see microscope objective lenses.
What happens when you mix them
Infinity objective on a 160 mm microscope
There’s no tube lens, so the objective has to form the image by itself. It can only do that if it’s moved slightly farther from the specimen than its designed focus position. In practice:
- You need to focus to a different position from your other objectives, so the set is no longer parfocal.
- Magnification won’t match the number on the barrel.
- The objective is working at distances it wasn’t corrected for, so aberrations, particularly spherical aberration, increase. At low power this may be barely noticeable. At 40x and above, the image usually softens.
160 mm objective on an infinity microscope
Now there’s an extra lens (the tube lens) in a path the objective wasn’t designed for. To compensate, the objective has to be moved closer to the specimen. This eats into the working distance, which is already tiny on high-power objectives. Depending on the design, you may not be able to reach focus at all, or you’ll reach it with degraded image quality and a magnification that doesn’t match the label.
Summary table
| Combination | Can you focus? | Parfocal with others? | Magnification correct? | Image quality |
|---|---|---|---|---|
| 160 mm objective on 160 mm microscope | Yes | Yes (within the same series) | Yes | As designed |
| Infinity objective on matching infinity microscope | Yes | Yes | Yes | As designed |
| Infinity objective on 160 mm microscope | Usually, at a shifted position | No | No | Often acceptable at low power, poorer at high power |
| 160 mm objective on infinity microscope | Sometimes; may run out of clearance | No | No | Usually degraded |
| Infinity objective on another brand’s infinity microscope | Usually | Possibly not | Possibly not | Varies; may show color errors |
Compatibility traps beyond finite vs infinity
Even within one type, several other things need to match:
- Tube length variants: most finite objectives are 160 mm, but some older makers used other values. Leitz, for example, long used 170 mm. Some metallurgical objectives use other tube lengths as well.
- Parfocal distance: 45 mm is common on traditional objectives. Some modern infinity systems use 60 mm. Mixing them breaks parfocality on the nosepiece.
- Threads: older and many budget objectives use the RMS thread. Several modern systems use larger metric threads such as M25 or M27. Adapters exist, but they don’t fix optical mismatches.
- Where color correction happens: some systems leave a little lateral color in the objective on purpose and correct it in the eyepieces (compensating eyepieces) or in the tube lens. An objective from such a system can show colored fringes toward the edges when paired with optics from another.
- Coverslip correction: 0.17 mm objectives are for covered slides, and objectives marked 0 are for uncovered specimens. This applies equally to finite and infinity designs.
Adapters and workarounds
Correcting lenses that let one type of objective work on the other system do exist in some optical catalogs and used-parts listings. Results vary, they can shift parfocal distance, and the cost can approach that of a suitable objective. For most amateurs, buying objectives that match the microscope is simpler and gives better results.
Using either type on a camera
Hobbyist photomicrography setups often mount objectives directly on a camera:
- A finite objective can be used with extension tubes or bellows, placing the sensor at roughly the designed image distance from the objective. No extra lens is needed.
- An infinity objective needs a tube lens in front of the camera. People commonly use a suitable achromat or a telephoto camera lens focused at infinity. This makes it easier to change magnification by changing the tube lens.
The same rules apply here: performance is best when the image distance or tube lens is close to what the objective was designed for.
Which should an amateur buy?
- For an existing microscope: buy objectives of the same type as your current ones, ideally from the same maker and series, so they stay parfocal. Our eyepiece guide covers matching eyepieces too.
- For a budget or used setup: 160 mm finite microscopes and objectives are widely available second-hand (see our used microscope checklist). For ordinary viewing they’re perfectly capable.
- If you want to add fluorescence, DIC or several accessories later: an infinity-corrected microscope is the more flexible platform, but plan on staying within one maker’s system.
To see where the objectives, tube and eyepieces sit in the overall instrument, see our guide to the parts of a compound microscope. For clearance differences between objectives, see objective working distance.